A wide variety of furnaces whose geometry, procedure and heating systems differ significantly are used in melting and treating metals and metallic waste. Depending on their mode of operation, the furnaces can be grouped into continuous or batch furnaces, which can use electricity or fossil fuels. They may also be classified according to their geometry. They can be of direct or indirect application. The advantages of each type of furnace are directly related to the type and size of the load used, since it largely determines the energetic efficiency and metallurgical quality resulting from the melting or treatment processes.
Moreover, one aspect common to all melting or treatment processes is the formation of floating slag. The way and condition in which the slag is separated from the molten or treated metal is a particular and distinctive feature of each furnace, as it represents an important restriction with respect to the operating system used. Therefore, whilst in the cupola furnace slag is automatically extracted continuously and in a liquid state, in an induction furnace it must be removed in a semi-solid state by manual and batch operation after each melting or treatment and before emptying the furnace. In rotary furnaces, this is done after the full tapping of the metal, by tipping or turning the furnace prior to proceeding with the new load.
In any case, the industrial reality presents a variety of furnaces with significant differences in performance and operability. The systems mostly used are based on direct heating of the load by means of induction currents, radiation or convection. The cupola furnace is an example of continuous direct heating and melting producing an excellent metallurgical quality, but it has the disadvantage of being a highly polluting facility as it uses coke as an energy source. Furthermore we must consider the quality and dimensional constraints imposed on the load in order to provide it with sufficient permeability and composition to allow the flow of ascending gases and the appropriate degree of recarburation. The electric furnace is not subject to these constraints, as it can take any type of load, with its size being the only limitation imposed by the diameter of the furnace. For example, European Patent EP0384987B1 describes an electric furnace. However, electric furnaces have the disadvantage of having to cool the coil, which represents a significant reduction in its energy efficiency and a high maintenance cost due to the high power factor to be contracted. Gas furnaces, despite using a less burdensome energy source, have even lower energy efficiency and cause higher losses through the oxidation of the load material due to convection heating.
The U.S. Pat. Nos. 4,060,408 and 4,322,245 describe reverberatory furnaces in which the metal bath surface is separated into different chambers. The metal is circulated using rotary pumps that propel it through passages and ducts made in the walls separating the different chambers. In both cases, the heating is direct and gas burners are applied both in the loading and the maintenance chamber, which leads to the inevitable oxidation of part of the metal and results in poor energy efficiency. The US patent application US2013/0249149A1 tries to solve this problem by mounting a radiant plate separating the load from the burner. Metal heating is produced by radiation of the plate on the metal bath protected by a nitrogen atmosphere to prevent loss caused by oxidation. However, the above three proposals are limited by the same aspect, that is the variable level of the height of the bath, which prevents the continuous removal of the generated slag. This imposes the performing of manual and repetitive cleaning, which interferes in the working of the furnace. For example, the de-slagging gates must be opened in the middle of the melting process.
Moreover, the mechanical arrangement of rotors immersed in the metal for its recirculation limits the use of these furnaces to non-ferrous metals of low melting point, not being suitable for processing iron or steel, whose melting point occurs at temperatures that the rotors submerged in the metal do not tolerate. For example, U.S. Pat. No. 8,158,055B2 describes a magnetic rotor coupled to an outer channel connecting two ends of a vessel and which generates a metal stream that extracts and reintroduces a small portion of molten metal into the heating chamber. This magnetic rotor cannot be used for recirculating all the molten metal, but is used to homogenize the bath temperature and chemical composition.
European patent application EP2009121A1 describes a waste treatment method in which a molten metal bed continuously moves and defines a closed circuit. The waste is retained on the surface of the molten metal bed. The waste is treated under the effect of the constant and continuous heat exchange generated by the movement of the molten metal bed beneath the waste retained thereon.
In sum, currently there are no furnaces of discretionary use (that is to say, which can be stopped and restarted at any moment, even when it is full with molten metal), in which the chemical composition can be modified at will thanks to the available access to the clean metal—for instance for adding an alloy forming metal—, which permits continuous removal of slag and which can be loaded with any dry metallic waste, while providing an optimized energetic performance.